The purpose of this study is to determine which of three combinations of antiretroviral therapy provides greatest reduction in viral load in a cohort of HIV infected children who are clinically stable. Safety and tolerance based on both clinical and laboratory toxicities will also be monitored. At the end of the study, it is expected that we will be able to recommend one of these combinations as most appropriate for phase III studies.The three combination therapies are the following: ZDV plus 3TC;D4T plus Ritonavir; and ZDV plus 3TC plus Ritonavir.